XxxHOLiC manages to give a Compressed Vice to a character who only appears in two episodes. After Watanuki manages to help convince a shy girl that her negativity is cursing her to fail and that she should try to be more positive, her more upbeat and outgoing twin suddenly turns into the sister from hell, psyching her out even worse than she ever did to herself until the poor girl is on her knees and paralyzed by the feelings of uselessness her sister is laying on her. Then, after Yuuko intervenes and the Aesop is learned, all is sunshine again.
Himitsu no Akko-chan, the original 1969 series, manages to wish a Compressed Vice to the main character, just to scare her into her personal Aesop. In episode 32, aptly named "_____", upon meeting a deaf-mute kid, Akko-chan, out of empathy and curiosity, wishes to her magic mirror to be a deaf-mute version of herself. Upon discovering that, being speechless, she can't wish anymore, and she'll be stuck that way forever, Akko-chan breaks apart, feeling scared and useless until the mirror, reasoning that she got her Aesop about hasty wishes and physical ailments, and she understood the true courage of her new friend (who will never be seen around for the rest of the series), lifts the wish on its own accord.
On more than one occasion in the Pokémon anime, Ash has gotten so full of himself specifically to get a Break the Haughty moment by the end of the episode, and then go back to being a reasonably humble trainer afterwards. Instances of this include his battles with Prima, Brawley, and Drake of the Elite Four. May also got this in one of her contests when she gained a Coordinator Superiority Complex out of nowhere and was repremended for it, and then it never comes up again.
In Blackhawk #240 (which is towards the end of the New Blackhawk Era), André Blanc-Dumont has been given a crippling fear of beautiful women. He declares himself cured after punching out a man disguised as a woman. Click here for an in-depth recap.
Many authors who worked on Iron Man gave Tony Stark's alcoholism a spin of their own, thus making him a borderline example. This trope applies largely because it is always restricted to specific plotlines. Outside these plots, he may be seen drinking but is never shown having this habit as a problem. Nevertheless, he is somewhat well known for this aspect of his character and there's no guarantee he won't go on a drinking binge again whenever someone decides they can make an innovative take on it.
Live Action TV
An episode of The Golden Girls revealed that Rose has been addicted to prescription strength pain-killers for decades. It also strongly implied that her perpetually sweet disposition is at least partially the result of taking these drugs. Despite the coda of the episode having her statement that she'll be fighting this addiction the rest of her life (albeit filled with hope that she can pull it off), it's never truly referred to again. Similar events happened to Dorothy, who had two relapses of former addictions she had beaten (smoking and gambling.) Aside from the episodes in question, they were never mentioned again.
Joey from Blossom hates a gay guy in one episode, revealing a prejudice that hadn't previously been mentioned in the show. Later in that episode, his black sister-in-law tells him a story about how she faced discrimination as a child, causing him to renounce his prejudice as quickly as he developed it.
A particularly offensive episode of Lizzie McGuire featured her pal Miranda becoming anorexic and then getting over it within the course of a week. It also had Gordo becoming addicted to Deeandeeaproximine...and then getting over it within the course of a week.
An episode of Spin City had Carter trying to quit smoking, despite having never been seen touching tobacco before (or since). This episode also featured Paul getting addicted to nicotine gum.
Ned's Declassified School Survival Guide, "Guide to School Records"- Ned is a well-intentioned, "smart butlazy" kid in the rest of the series, but this episode shows him pulling all sorts of deliberately mean pranks on his way to accruing the biggest permanent record in the school. Flashbacks are used, which (unusually for the show) were filmed just for this episode, not taken from earlier ones, further playing up the trope.
If a young attractive female character is introduced to a series and some fuss is made over the "fact" that she smokes, then it is near certain that that will be the last time that she is seen with tobacco, or that it will even be mentioned. Examples-
Marissa Taylor in the defunct Australian comedy/drama Always Greener. Admittedly this last one could be regarded as just a set-up for a joke about an exploding cow, but credibility was stretched in a later episode where she stood right next to another character who was smoking, without batting an eyelid.
A particularly extreme example appeared on Rome, with the reveal that Octavian was deeply in love with (as in, wanted to have sex with) his own sister. Not only had nothing even hinting about this ever come up before, but the episode itself has zero hints about it until Servilia lets his sister know— which actually justifies it, as he was clearly very good at keeping it secret.
In the first episode where we see Chandler smoke and the others disapprove of it, he delivers a speech about how he accepts their flaws and only expects them to accept his in return. Said flaws include Joey cracking his knuckles, Monica snorting when she laughs and Phoebe chewing her hair — none of them appeared before, or after this episode. The one about Ross overpronouncing every word applies, though.
A Friends episode that shows less respect for continuity comes in the Season Five New Year's episode where Rachel suddenly turns into a gossip who can't shut up about her coworkers' dirty laundry. The whole thing turns out to be a plot device to launch us into a Cold Turkeys Are Everywhere plot as Rachel resolves to stop gossiping, and then immediately discovers the unkeepable secret that Chandler and Monica are doin' it.
Aesops about snoring tend to suffer from this, as characters spontaneously develop the habit and then no reference is made to this afterwards. Examples include Joey from Friends (Chandler can suddenly hear him through the wall after living with him for five years with no problem), Charles Winchester from MASH (due to allergies), and Homer Simpson from The Simpsons (he suddenly starts snoring loudly after years of sleeping with his wife).
Done in Red Dwarf where the crew is forced through the air ducts of Starbug. Lister is revealed to have claustrophobia. Subverted somewhat when Cat lists a number of examples where he's been trapped in a confined space and didn't freak out, naturally this didn't help Lister.
Star Trek: The Next Generation: Worf was the victim of this in the episode where a genderless alien species showed up, and he was saying things like it being "unnatural" and the like. This particular prejudice wasn't seen previously in all the cases where he met aliens who didn't have a traditional gender setup, and never appeared again.
When Odo becomes involved with the FemaleChangeling in "Behind the Lines" and "Favor the Bold," he becomes utterly obsessed with linking. Given that their linking scenes have the feel of love scenes, and that he neglects other duties to link with her, his actions seem to be a metaphor for sex addiction. Oddly enough, he never demonstrated this kind of obsession with linking when he melded with fellow Changelings before or afterwards.
There was a late episode, where there was a homicidal shapeshifter on the station, and the other main characters responded by revealing their prejudice against shapeshifters, which had never been hinted at before, even after years of fighting a Dominion run by shapeshifters. Might have been partly explained by that particular shapeshifter constantly harping about how everyone else was prejudiced against him for being so superior to them (which inclined them to treat him like a jerk). Odo's friends make an effort to be nice to the stand-offish stranger at first, but he brushes them off and accuses them of trying to make Odo an Uncle Tom.
Similar example in Star Trek: Voyager, in an episode where the Doctor found out that Janeway had tampered with his memories to prevent him going "insane" over an old triage case, and Janeway and the entire crew suddenly seemed to develop an anti-AI prejudice which then immediately vanished again next episode.
Made worse because another episode had them arguing the Doctor was human, not just an AI, when he was denied rights over the publishing of his holonovel for being a hologram.
Tommy from 3rd Rock is revealed in one episode to have been hiding sandwich bags full of spices to indulge his secret cooking hobby in secret ("It's marijuana, I smoke it with friends I swear!"). This is never mentioned again.
MASH: the plot of episode "C*A*V*E" is based on Hawkeye's suffering from crippling claustrophobia, which had never been mentioned before and was never referred to again.
Likewise Colonel Foster in the UFO episode "Sub-Smash".
The Professionals. In "Klansmen" Bodie displays overt racist behaviour never shown previously by his character, and due to the events of the episode (in which his life is saved by a black doctor) we never see it again. Actor Lewis Collins was not pleased.
Subverted in the Malcolm in the Middle episode where Francis turns out to have been in AA despite never having been shown getting drunk in previous episodes. The other characters find out that he had all the signs of alcoholism except for drinking.
An episode deals with the annoying habits of the group. The bad habits of Ted, Marshall, Barney and Robin are noticeable prior to the episode (although Robin's misuse of the word was subtle before it was pointed out), and they still have them in later episodes. Lily's habit of chewing too loudly is a true Compressed Vice, as it appeared only for that episode.Justified in later episodes featuring the characters throwing "interventions" to stop each other's similar minor annoying habits: though Barney's use of magic was featured in previous episodes, other character's habits had just never been incorporated into Future Ted's unreliable narration.
Another episode shows all five characters being habitual cigarette smokers. Previously, Barney and Robin had been seen smoking cigars, and it was hinted that Robin smoked cigarettes, but this episode portrayed Robin as practically a chimney. The other characters don't smoke nearly as often, but obviously way more than has ever been let on before. Ted's children are stunned at the news. Possibly justified, if you look at the on-screen events of the show as literally being Future Ted's retelling of his past and that he omitted any mention of the gang smoking until that episode.
In the Lent episode of Father Ted, while Ted's smoking and Jack's drinking have been previously established, Father Dougal's addiction to roller blading only exists in this episode. However, as the whole series runs on Negative Continuity and Rule of Funny, this scarcely seems to matter.
An odd example from 24 in that Jack's heroin addiction from season three is dealt with over multiple episodes, but since those episodes take place over one day, he really should be suffering for far more than the first few hours. But then, many examples can be taken from the show where people get over things (emotionally or physically) way faster than they should realistically be able to - Tony having major surgery after being shot but getting straight back to work just a couple of hours later, for example. The heroin thing was handwaved by Jack being given some vague other drug that would mask the withdrawal symptoms for about a day, i.e. the rest of the season, after the writers realized it was becoming more trouble than it was worth.
Eri in Tensou Sentai Goseiger is revealed to be very messy and lazy in Epic 9, causing her to clash with Moune as part of their focus episode. These bad habits are never mentioned before or since.
For the Glee episode "The Power of Madonna", the boys are suddenly shown mis-treating the girls in various ways to a highly exaggerated extent, in order to setup the feminist message of the episode. This is incredibly jarring because, for instance, Artie is shown being rude and misogynistic to Tina, even though he has never displayed this attitude before.
Stumpy's gambling addiction isn't mentioned at all in season one of Carnivŕle, even though by the beginning of the second series he has the debt collectors after him and a $400 debt (in old-timey Great Depression-era money). Adjusted for inflation, $400 in 1934 would be worth about $6443.73 in 2010's dollars.
In one Very Special Episode of Boy Meets World, Shawn gets drunk for the first time and then has a drinking problem for about a week before his friends convince him to give up drinking altogether. However, he does turn back to alcohol in an episode two seasons later after he learns some devastating news, though only for that episode.
In one episode of Alice, Alice, Flo, and Vera all try to help each other kick their previously-unmentioned vices: Alice eats too many sweets, Flo drinks too much coffee, and Vera very uncharacteristically smokes. None of these vices, or the fact that at the end of the episode, they had all switched vices, was ever mentioned again.
In the Community episode "Regional Holiday Music", Glee Club instructor Mr. Rad insists that Britta play the part of a mute tree, and when we finally see Britta sing her awkward song, we understand why - she's terrible. Thing is, we've heard Britta sing in other episodes. We hear Britta sing in the very next scene. She's not terrible at all unless the plot requires it.
The The Mary Tyler Moore Show episode "Mary's Insomnia" has Mary turning to sleeping pills to get over a new-found case of insomnia, becoming dependent on them, and getting over her addiction, all within one 25-minute episode.
Samantha in Bewitched experienced pregnancy-related food cravings in a single episode: "Samantha's Curious Cravings".
Subverted in It's Always Sunny in Philadelphia. Dee and Dennis spend an episode addicted to crack, but by the end they're off to a recovery program. In later episodes, no mention is made of their previous addiction... until "Frank's Pretty Woman," where they encounter crack again. Dennis immediately flees the scene, saying that it's not a safe place for him, then soon afterwards convinces Mac that crack is awesome and they should go get some.
Inverted and possibly subverted in the webcomic Narbonic, where Dave's chain-smoking habit is established early on and continually referenced. However, after Dave goes back in time and alters the event that causes him to start smoking, he is surprised to find that he has no addiction at all... and the other characters assure him he never did, smoking was never relevant to any of their adventures, and they are confused when he brings it up. The author even devotes a filler comic to two fans explaining how the previous plots where his habit was a key point make sense without it.
Subverted in Unshelved. A storyline deals with Colleen quitting smoking — when there was no indication of her being a smoker before, and the other characters are surprised to hear about it. At the end of the storyline, it turns out this is because she quit decades ago, when she was still a teenager — she made it sound current as an excuse for being rude to a patron at the library.
The Garfield and Friends episode "Sales Resistance" revolves around Garfield's obsession with buying useless stuff off the Shopping Channel - an obsession which he has only in this episode.
Danny Phantom does this frequently, with Danny himself being the usual suspect.
Tucker had one of these in "Doctor's Disorders." He had a horrible fear of hospitals that we'd never seen or heard of until he had to wear a paper bag over his head just to walk past the nurse's office.
The Simpsons has been doing this every week for the past decade. Though some have come to receive Continuity Nods every now and then. One memorable aversion came with Marge's gambling problem, which was expected to become forgotten by the next episode like most but has been managed to be referenced countless times. In some cases, outside of the show.
One of the most glaring examples of this in the episode "Fear of Flying", which suddenly introduced Marge's titular phobia and linked it to several incidents in her childhood despite an earlier episode ("Mr. Lisa Goes to Washington") having had the family go on a flight with no incident whatsoever.
One episode had Bart's hellion tendencies stemming from Homer's inability to punish Bart for his bad behavior. The same Homer who routinely strangles him for misbehaving.
Which was then the focus of another episode when he went to a parenting class because strangling Bart as punishment was inappropriate.
Both Homer and Marge had separate displays of homophobia, punctuated even further by both of them showing the opposite opinion in each other's bouts.
One of the most horrid cases takes place in the later seasons of Static Shock. In one of several Very Special Episodes, Adam Evans (aka Rubber-Band Man) out of nowhere suddenly has dyslexia, and from what we're told, has had it since he was a child. And not just a mild case for that matter, more like sees a stop sign written in Arabic type. The episode ends with Static and Adam giving a brief speech about dyslexia a la And Knowing Is Half the Battle and Adam's reading disorder is never brought up again. This is particularly egregious, since Adam was previously shown reading. In fact, one episode had him mention aloud (With nobody around for him to be trying to lie to) "I still have all this fan-mail to read" as he picks up, opens, and begins to read said mail (Even more of a retcon since when his dyslexia appears in this episode, the fact that none of his fan-mail was open becomes a clue that he could not read).
In one episode, Bender reveals that he cannot get up if he is knocked onto his back. By the end of the episode, he learns how to overcome this.
One episode gave Bender an obsession with being remembered - something he'd never even hinted that he might have had before. It never came up again.
In another episode, four of Bender's ten most frequently used words were words he only used in that episode. (Obviously, a parody.)
Hot diggity daffodil!
Bender's irrational hatred of Nibbler lasts exactly one episode.
In the beginning of "Neutopia" every male Planet Express employee is suddenly sexist, and all the female employees suddenly act like stereotypical women. It probably was just to get the plot going, but still.
In "Benderama" Bender is psychotically lazy, to the point that it's his only defining trait in that episode.
The human crew's sudden (and inconsistent) robot racism in Fear of a Bot Planet.
Fry: So let me get this straight. This planet is completely uninhabited? Bender: No, it's inhabited by robots. Fry: Oh, kinda like how a warehouse is inhabited by boxes.
Leela and Amy are suddenly extremely vitriolic and competitive towards one another in "The Butterjunk Effect", when they've got along fine in every other episode. Made even weirder by the fact that Fry states that they've always been this way.
In "The Prisoner of Benda" Amy is suddenly an extreme glutton.
To be fair, it has been stated a few times that in her younger years, Amy suffered from being noticeably overweight.
Haven't seen enough episodes to be sure, but an episode of The New Woody Woodpecker Show, "Automatic Woody", has Woody dreaming about eating "Butterscotch Finger Pies", and then waking up and finding tons of empty wrappers in the various places he's stashed some for midnight snacking, and then going through lots of trouble to buy some more.
Unfortunately, most of Kim Possible's Character Development is about getting a character flaw of the week that must be conquered by the end of the episode. This includes her being extremely competitive in one episode (never mentioned again) and telling white lies constantly (brought up but then presumably ignored, because it was never brought up again and she continued to do it).
Brenda's slovenliness was revealed in the same episode of Teamo Supremo as it was cured. (At least her desire to be a famous pop singer cropped up in more than one episode.)
The Producing Parker episode "The Skinny on Parker" had Parker developing anorexia and immediately getting over it after being force-fed a sandwich.
One episode of The Fairly Oddparents revealed that Trixie Tang was actually a tomboy who liked "boy things" but is embarassed to show that side of her to any of her friends in the popular crowd. Like a lot of things in this show brought up in just one episode, it was never mentioned again.
An episode of King of the Hill revolves around Hank's obsession with his guitar. Peggy claims that he pays more attention to the guitar than to her despite the fact that it's the only episode in which the guitar appears. It was eventually replaced with a similar vice: him treating the family dog Ladybird extremely well, sometimes better than he treats Bobby or Peggy. This one, however, stayed through the entire series.
She again does this when he was spending more time with Bobby who was doing well in Home Ec., though this was more from the longer-running character flaw of Peggy that she has to be the center of attention and acknowledged as the best at everything; Bobby taking up cooking was something she supported at first until it became clear he was better at it than her and getting more attention from Hank as a result.
They also had an episode in which the whole family took up smoking, and was over it by the end credits.
In an episode of Total Drama Action, Trent demonstrates an unhealthy obsession with the number 9, which he claims he's had since childhood even though he never showed any signs of it in the previous season.
An episode of Jimmy Two Shoes had Beezy have a severe cell phone addiction which is never mentioned before or after. He never gets over it, in fact his addiction saves everyone.
In one episode of Chip N Dale Rescue Rangers, Dale has the same reaction to chocolate that Monty usually has to cheese. It never really comes up before or after that mission.
In one episode of Slacker Cats Buckley becomes a gambling addict and there's no notion before that he has that sort of characteristic and it never happens again.
An episode of American Dad reveals that Francine has a hatred of left-handed people. This is because she was originally left-handed as a child, but a nun she was raised by was convinced that lefties were evil and disciplined her into using her right. At the end of the episode when Steve and Hayley get her to accept the fact she's left-handed she tries to get used to being a lefty again and handles tasks like you would expect a right-handed person doing tasks with their left for the first time would do, from writing on the shopping list illegibly to slitting Steve's throat with a butter knife (he got better).
Also, "The Wrestler" has Stan obsessed by his wrestling record from high school, with a room in his house devoted to the trophies, and Francine is sick of hearing about his wrestling career and touring his museum. Odd, then, that none of the previous 126 episodes had mentioned any of this. Lampshaded by Steve commenting that he's lived in the house all his life and has never seen the wrestling museum before.
"Pulling Double Booty" suddenly reveals that Hayley has severe anger issues when being dumped (by "severe", I mean "rampaging death machine"), even though she was dumped by an illegal immigrant in "American Dream Factory" and harmed nobody (though she did call the FBI to capture him), and yet the episode implies she's been The Berserker since she was a child.
One could argue it was a reference to her recurring Hypocrite role, giving her tendency to go through men rather coldly, it seems perfectly in character for her to cry hell fire when someone does it to her.
In Madagascar: Escape 2 Africa, Melman the giraffe suddenly develops deep feelings for Gloria the hippo. Nothing of the sort was mentioned in the first film. A flashback at the beginning of the second film shows that, even as children, Melman had a crush on Gloria. Additionally, an inverse case with Melman's hypochondria in the first film, which is completely gone by the sequel. The only hint appears to be his great knowledge of medicine.
The Powerpuff Girls go through this in "Candy is Dandy." They suddenly become addicted to candy, so much that they actually get their enemy, Mojo Jojo to supply it to them. When Mojo stops the supply, the girls beat him within an inch of his life, only for Blossom to finally wake up and stop the beating after seeing the monsters they've became.
Justified with Stan's dad's single-episode struggle with alcoholism. He was pulled over for Drunk Driving and forced to attend AA despite not really being an alcoholic. Impressionable joiner that he is, he takes the "admitting you are helpless to help yourself" part rather too readily to heart, and descends into full-blown dependency, convinced he has a terminal illness that forces him to drink.
Adventure Time's Finn falls into the dark, foul abyss that is shipping addiction in "All the Little People". While it only lasts an episode, there's a FOUR MONTH timeskip in the middle, in which Jake moves out after finding Finn's behavior creepy.
Another episode randomly gave Jake life-threatening levels of ADHD, which are never brought up before or after.
Goofy is randomly a Big Eater in the Goof Troop episode "The Incredible Bulk." Despite this being a large part of the plot of this episode, it is never brought up before the episode or again and other episodes portray Goofy more logically as a light eater. The other characters who are more consistently played as Big Eaters (Pete and PJ to a lesser extent) are shown to be astonished by his appetite in this episode too.
Rufus of The Dreamstone spent the majority of the series as an almost sickly sweet Hero Antagonist, however the rare occasion one of the heroes was required to say or do the wrong thing and get handed An Aesop, he would usually take the role. Amberley would sometimes join in on this as well.
Sonic Rainboom takes Rarity's vanity to an extreme that results in her focusing more on showing off her wings than on helping Rainbow Dash. note (Rarity's wings end up falling apart mid-flight; furthermore, Rainbow Dash is the one who rescues her from hitting the ground.)
Ponyville Confidential involves the CMCs being so insistent on making it big in the school newspaper that they're willing to gossip about their own siblings. note Eventually this backfires on them when Diamond Tiara blackmails them with embarrassing photographs of themselves; they decide the best way to deal with this is to come clean to everyone about being the source of the gossip, and by that time the teacher removes Diamond Tiara's authority in the school newspaper anyway.
A more threatening temporary villainy occurs during Secret Of My Excess, when his greedy dragon nature gets out of control. note When Rarity, not knowing he is the same Spike, talks about how generous Spike was, he feels ashamed of what he has become and immediately reverts back to his old self.